Recently, demands for various kinds of films for industrial use have been increased, and production of films having excellent heat resistance and mechanical properties has been required. Particularly, films for magnetic tape base has been expected to be highly improved in mechanical strength, especially, mechanical strength in machine direction. Generally, in plastic films, thinner films are required to have better processability and sliding properties. Particularly, as for industrial films such as magnetic tapes, techniques which mainly comprises addition of particles to various resins to satisfy both requirements of smoothness and sliding properties have been developed. However, with the recent spread of video tapes, much smaller video tapes with higher picture quality which can record for longer period of time have been required, and techniques by depositing magnatic materials, particularly by vertical magnetization have attracted attention and developed. In such techniques using deposition, surface of the base film is required to have high smoothness and heat resistance. Further, with recent densification, films with better smoothness have been required.
Among them, some films of polyethylene terephthalate (PET) which satisfy high smoothness and sliding properties have been developed with the progress in film-forming technique. However, they have problems, for example, poor heat resistance during deposition of magnetic materials, separation of oligomers, hydrolysis under high humidity and dimensional change and the like, further the tape may be stretched at a temperature over its glass transition temperature, producing problems in the usage in a car, where the tape may be exposed to a higher temperature.
Recently, styrene polymer having a high degree of syndiotactic configuration has been developed (Japanese Patent Application Laid-Open No. 104818/1987). A film made of said styrene polymer is excellent in heat resistance, dimensional stability, electrical insulating properties and the like, and expected to be widely used, particularly favorable as an industrial film.
When a film is produced using this styrene polymer having a high degree of syndiotactic configuration as a raw material, its mechanical strength in machine direction (MD) can be greatly improved by uniaxial stretching, whereas it tends to be torn in MD direction. Production of the film by the conventional simultaneous or sequential, biaxial stretching results in improvement of mechanical strength in all directions, providing relatively well balanced film. However, attempts to improve mechanical strength in a specific direction may be inevitably accompanied with deterioration of strength in the direction vertical to said specific direction. Accordingly, it was difficult to produce films which completely satisfy mechanical strength in MD required for films for magnetic tape bases.
Japanese Patent Application Laid-Open No. 110122/1989 discloses a process for the production of a film of styrene polymer having a high degree of syndiotactic configuration. However, it is difficult to greatly improve mechanical strength in MD by this method, which is particularly utilizing simultaneous, biaxial stretching with a tenter frame by batch process. Further, production efficiency of the stretched film is insufficient. Moreover, it is difficult to continuously produce product with uniform quality.
As for production process of a film by calendering, especially a process which comprises calendering a composition of the above styrene polymer having a high degree of syndiotactic configuration, concrete calendering operations, conditions and the like have not been established.
The films of styrene polymer having a high degree of syndiotactic configuration which are produced by such conventional methods have problems in mechanical strength, as mentioned above, and further have insufficient smoothness and sliding properties. Films having such physical properties and excellent in surface tension have not been developed.